Electrolytic recording systmes

ABSTRACT

THE INVENTION PROVIDES PROCESS OF ELECTROLYTIC RECORDING IN WHICH AN ELECTRIC CURRENT IS PASSED BETWEEN AN ANODE CONTAINING METALLIC SILVER AND AN INERT CATHODE THROUGH A POROUS WEB IMPREGNATED WITH AN ELECTROLYTIC CONDUCTING DISCLOSURE TO PRECIPITATE METALLIC SILVER IN SUCH WEB AND IN WHICH SAID ANODE ALSO CONTAINS A SECOND METAL SELECTED FROM MAGNESIUM, MANGENESE, ZINC, CADMIUM, NICKEL AND ALUMINIUM AND/OR SAID SOLUTION A SOLUBLE SALT OF SAID SECOND METAL.

United States Patent ELECTROLYTIC RECORDING SYSTEMS Robert D. Richards, Chatham Township, N.J., assignor to Muirhead Limited, Beckenham, Kent, England No Drawing. Filed Oct. 31, 1968, Ser. No. 772,430 Claims priority, applicatior; grgait Britain, Nov. 6, 1967,

Int. Cl. B41m 5/20 U.S. Cl. 204-2 13 Claims ABSTRACT OF THE DISCLOSURE The invention provides a process of electrolytic recording in which an electric current is passed between an anode containing metallic silver and an inert cathode through a porous web impregnated with an electrolytic conducting disclosure to precipitate metallic silver in such web and in which said anode also contains a second metal selected from magnesium, manganese, zinc, cadmium, nickel and aluminium and/ or said solution a soluble salt of said second metal.

This invention is concerned with improvements in and relating to electrolytic recording systems and materials for use in such systems.

Electrolytic systems of recording are well known. Typically, a paper impregnated with a solution (normally of a phenolic substance) is drawn between an iron-alloy anode and a noble metal cathode. Passage of an electric current causes iron to pass from the anode to the paper, where it reacts with a constituent of the solution to form a dark-coloured precipitate, so marking the paper. Methods of increasing the intensity and improving the colour of such systems are described in our British Pats. 786,496; 879,864; 856,862.

Such marking systems are effective at low recording speeds (up to around 100 ins. per sec.), but *fail at higher speeds, where the mark becomes weak and diffuse.

Also well known, but of little utility, is the production of similar marks by the electrolysis and subsequent reduction of silver. Although this has obvious interest because of the close analogy with photographic processes, the amount of silver which can be introduced to the paper is small. At practical rates of recording the mark is weak and fugitive. The silver quickly coalesces, with a pronounced reduction in the intensity of the mark.

We have found the new and unexpected result that if silver is reduced in the presence of oxides or hydroxides of certain other metals the mark is in many cases strong and permanent.

Consequently, in accordance with one aspect of our invention We provide a method of writing marks on paper or other porous webs in which a silver salt is reduced in the presence of an oxide or hydroxide of certain metals to be herein disclosed.

The metal which forms the oxide or hydroxide may be selected from the class:

Magnesium, manganese, cadmium, zinc, nickel, aluminium.

We have further ,found that silver oxide itself will function as the oxide, though this is not preferred as the amount of reducing agent must be arranged to precipitate only part of the available silver as the metal.

Such a method can be employed as the basis of an electrolytic recording system which is capable of greater definition, greater intensity, and a greatly improved writing speed, which constitutes another aspect of the present invention.

Accordingly the invention provides a process of electrolytic recording in which an electric current is passed ice between an anode and a cathode through a porous Web impregnated with an electrolytic conducting solution and a silver salt in said solution is thereby reduced to metallic silver in the presence of an oxide or hydroxide of a second metal selected from magnesium, manganese, cadmium, zinc, nickel and aluminium.

In analytical chemistry a well known test for manganese is to precipitate simultaneously manganese oxide and silver oxide. The product is an intensely black precipitate, more intense than either oxide precipitated separately. The effect has been ascribed to the mutual reduction of silver and manganese, but we have found that this effect is not confined to these metals.

When silver is precipitated from a solution of its salts by the action of a reducing agent; the precipitate is at first black, but rapidly lightens in colour to a pale grey. The change in colour is due to colloidal particles of silver coalescing to form larger particles. When an oxide or hydroxide is precipitated at the same time as the silver, in many cases the black colour is retained. We ascribe this effect to the separation and protection of the colloidal silver particles by the oxide or hydroxide. In electrolytic recording papers the effect is greater than in vitro, due probably to the fibres of the paper assisting in the separation of colloidal silver.

Precipitation of the oxide or hydroxide can be effected either by electrolysis of the metal into a solution which has a pH value greater than that at which the oxide or hydroxide is precipitated; or by including in the solution a soluble salt of the metal, and allowing an increase in pH value to occur during electrolysis.

The invention may be applied to electrolytic recording systems in various ways. For example, the silver may be introduced as the anode material, or alternatively as a salt e.g. as a nitrate in the solution with which the Web is impregnated and is suitably present in an amount of up to 5% by Weight in the solution. The latter is not preferred, since the sensitivity of silver salts to light greatly reduces the utility of the system.

Similarly, the second metal may be introduced as the anode material, or as a salt in the solution. Again both metals may be introduced as the anode material, in which case the anode may be an alloy of the two metals or it may comprise contiguous pieces of the individual metals, provided both metals are in contact with the paper.

Thus, in accordance with another aspect of the invention there is provided a process of electrolytic recording in which an electric current is passed between an anode containing metallic silver and a cathode through a porous web impregnated with an electrolytic conducting solution to precipitate metallic silver in said web and in which said anode also contains a second metal selected from magnesium, manganese, zinc, cadmium, nickel and aluminium and/or said solution contains a soluble salt of said second metal.

The solution may contain, in addition to the salts of silver, and the other metal when necessary, an additional electrolyte to assist in conducting the electric current through the paper. This electrolyte, by the electrolytic reduction of its cation at the cathode, produces an alkaline substance which increases the: pH value of the solution, and assist the precipitation of the oxide or hydroxide. The chlorides, nitrates and sulphates of the alkali metals and of ammonia are preferred, potassium nitrate and sodium nitrate being particularly preferred.

The additional electrolyte is suitably present in the solution in an amount of from 5 to 15% by weight, preferably in an amount of about 10% by weight.

The pH of the solution suitably lies within the range 7-11, preferably 810.5, and to this end the solution may contain one or more alkaline substances and/or alkaline buffering agents. Alkaline substances include alkali metal and ammonium hydroxides such as potassium or silver hydroxide. The alkaline buffering agents which may be used include the formates, acetates, carbonates, bicarbonates, borates, phosphates, tartrates and the like of the alkali metals and the alkaline earth metals, sodium and potassium carbonate being preferred. The alkaline buf fering agent is suitably present in the solution in an amount of up to 25% by weight, preferably in an amount of 0.1 to 1.0% by weight.

The porous web may be a woven or unwoven fibrous web and is preferably a paper web. However, other webs such as closely woven webs of synthetic or natural fibres may be employed. The web is suitably impregnated with from 25 to 60% by weight of the solution. This impregnated web is provided as a further feature of the invention.

The solution may also contain a reducing substance to assist in reduction of silver ions when the metal is manganese, nickel, aluminium or magnesium. Where the hydroxide does not possess reducing properties this reducing substance is essential. It may be selected from any of the well-known photographic developing agents, but we prefer those which are colourless in solution at the pH values at which they are eifective. In particular we prefer to use the aldehyde sulphoxylate reducing agents; e.g. the formaldehyde sulphoxylate reducing agents.

The metal of which the cathode is composed does not enter into the essential reactions, but merely provides electrical contact with the paper. However, it is desirable that it should not be attacked by the solution. Suitable metals for the cathode are platinum and its alloys, steel and nickel.

The process of the invention may suitably be used in conventional electrolytic facsimile recording apparatus such as that of the drum and helix type such as is illustrated in US. Pats. Nos. 2,063,992 and 2,202,855; provided, of course, that the anode of the apparatus contains silver and, if necessary, a second metal in accordance with the invention.

As stated above, the impregnated webs used in the process of the invention are presented as a further feature of the invention and those containing no reducing agent possess the advantage that they have a considerablp longer shelf life, e.g. about 2 /2 years as compared with about 6 months, than those containing the reducing agent. This is believed to be due to the fact that reducing agents have an adverse effect on the paper whereas in the case of papers not containing reducing agents the life of the paper is limited by its own wet-strength life.

An example of a system in which silver is introduced as the anode material, and the hydroxide-forming metal is contained in the solution as a soluble salt, is

Anodesilver Solution:

Manganous nitrate-10 parts Potassium nitrate-30' parts Potassium carbonate-2 parts Water-259 parts Cathodenickel An example of a system in which both metals are introduced as the anode, is

Anode:

Alloy:

Silwer80 parts Manganese-20 parts Solution:

Potassium nitratel parts Potassium carbonate1 part Water-89 parts Cathode--platinum An example of a system in which the silver is introduced as the anode, the hydroxide-forming metal is introduced both as the anode material and in the solution, and the solution contains a reducing agent, is

Anode:

Alloy:

Silver-70 parts Magnesium30 parts Solution:

Sodium nitrateparts Magnesium formaldehyde sulphoxylate20 parts Sodium carbonate--l part Water-879 parts CathodcStainless Steel An example in which silver is only partially reduced,

AnodeSilver Solution:

Sodium nitrate50 parts Sodium glyoxal sulphoxide-10 parts Sodium bicarbonate1 part Water439 parts Cathodegold The above systems have been found to give good markings when tested.

I claim:

1. In a process of electrolytic recording in which an electric current is passed between an anode and a cathode through aporous web impregnated with an electrolytic conducting solution containing a silver salt in said solution, the improvement which comprises conducting the electrolytic recording in the presence of an oxide or hydroxide of a second metal selected from the group consisting of magnesium, manganese, nickel, cadmium, zinc and aluminum, to reduce said silver salt to metallic silver during the electrolptic recording process.

2. A process according to claim 1 wherein said silver salt is introduced into said electrolytic conducting solution by electrolytic action from an anode containing metallic silver.

3. A process as claimed in claim 1 in which at least a part of said second metal is introduced into said electrolytic conducting solution by electrolytic action from an anode containing said second metal in metallic form.

4. A process according to claim 3 wherein said anode comprises silver and said second metal.

5. A process according to claim 4, wherein said electrolytic conducting solution contains a soluble salt of said second metal.

6. A process according to claim 1 wherein said electrolytic conducting solution contains as electrolyte, a neutral water-soluble salt selected from the group consisting of chlorides, sulphates, and nitrates of alkali metals and ammonia in an amount from 5 to 15% by Weight, based on the weight of the solution.

7. A process according to claim 1 wherein said electrolytic conducting solution has a pH of from 7.5 to 11.

8. A process according to claim 1 where said electrolytic conducting solution includes an alkaline buffering agent selected from the group consisting of formates, acetatcs, carbonates, bicarbonates, borates, phosphates and tartrates of alkali metals and alkaline earth metals.

9. A process according to claim 8 wherein said alkaline buffering agent is present in said solution in an amount of up to 2.5% by weight, based on the weight of the solution.

10. A process according to claim 1 wherein all the reduction of said silver salt to metallic silver is effected without any additional reducing agent.

11. A process according to claim 1 wherein said second metal is selected from the group consisting of magnesium, manganese, nickel and aluminum and a part of 6 said reduction is effect-ed by the action of an additional References Cited chemical reducing agent.

12. A process according to claim 11 wherein said re- UNITED STATES PATENTS ducing agent is an aldehyde sulphoxylate reducing agent. 2570096 10/1951 Calkm 13, A process according to claim 1 wherein said p0r- 5 3,053,179 9/1962 Relthel 2042X ous web is a paper web impregnated with from 25 to 3,402,109 9/1968 Berman et 60b ht, db',f'dltlt' fg gjfi' i on a W 0 eecmy PATRICK P. GARVIN, Primary Examiner 

